soc: bcm: bcm2835-pm: Fix PM_IMAGE_PERI power domain support.

We don't have ASB master/slave regs for this domain, so just skip that
step.

Signed-off-by: Eric Anholt <eric@anholt.net>
Fixes: 670c6726 ("soc: bcm: bcm2835-pm: Add support for power domains under a new binding.")